If they are drums right now, simply going to disc will make a big difference.

If they are already disc, there are a few options:

1) If you have a lift, you can get the stainless braided hoses, they have a smaller inner hose diameter, making the brakes more responsive (they are longer than stock, so no good if you have no suspension lift, and also available for the rear hose).

2) You can get high-performance rotors, with grooves in them, they provide quicker stopping power and better heat dissipation, they even have them in Stainless Steel.

3) If your master cylinder is manual, you can look into switching over to a power brake setup, buying the booster and everything else for the conversion.

4) You can buy the rear disc conversion kit, switching the rear drums to disc (pricey, and personally, my last option I would ever use, since rear brakes only supply 20% to 25% of your braking on average, I would put more money into front brakes or the power conversion).

I agree with 4wheeler,thats a good plan,another thing to think about is that rear drums suck in reverse! whereas a disk brake works equaly as well. Good for wheeling when you just cant quite make it to the top of that hill climb and need to back down slowly.
just my .2cents worth
Oh and just FYI, they do make DOT approved stainless steel brake lines.